In the current era, the development of infrastructure is indeed fast as many projects are still incessantly improving the quality of life. For building around the sea, it needs to be treated with sea water, and in addition to utilizing waste, fly ash is also useful as a cover for concrete cavities so that the strength of the concrete increases. On the other hand, in project work, it is often limited to a tight schedule, one of the casting jobs is dismantling formwork. The innovation in this research is analyzing concrete with material added bestmittel 0,6% and fly ash 5%, 10%, 15% content from the weight of cement by soaking sea water asainst compressive strength. Using a cylindrical test object measuring 150 x 300 mm and curing for 3 days, 7 days, 14 days, and 28 days, from the results of the compressive test on concrete at the age of 3 days, 7 days, 14 days, and 28 days, the average compressive strength was obtained with fly ash 5% in the order of 18,87 MPa, 24,24 MPa 35,19 MPa, 30,27 MPa. For fly ash 10% is 18,03 MPa, 25,57 MPa, 27,29 MPa, 36,98 MPa. For fly ash 15% is 17,89 MPa, 25,25 MPa, 28,71 MPa, 36,95 Mpa with a planned compressive strength of 35 Mpa. It can be concluded that it has met the compressive strength of the plan and the highest compressive strength is the highest is bestmittel 0,6% and fly ash 10% with sea water immersion of 36,98 MPa.